How migratory birds know exactly which route to take
Geo-magnetism, starlight and other cues that help migratory birds find their way
How do migratory birds know where they are going? Some of their navigational tools will be familiar to humans, while there are other fascinating cues we are still learning about. As humans, we sniff out our favourite fast-food restaurant just by following the smell. Perhaps we travel between two familiar places based on key landmarks. If we are on a road trip, we may even change the route based on what the group collectively decides. Birds use these strategies, too, but they can also get navigation cues from stars or the Earth's magnetic field.
The Earth's magnetic field is a giant, dynamic invisible shield around our planet that protects from the sun's harmful radiation. Researchers believe that birds and other long-distance migratory animals like turtles and insects might use the magnetic field to figure out where they are and where they should go. While birds may not have an internal compass with a red arrow pointing North, they might somehow detect the natural gradients in the magnetic field. There is even some reason to believe that a bird's "magnetic sense" is tied to other sensory systems like vision.
Most of what we know about birds and geomagnetism comes from experiments in controlled laboratory settings. When researchers manipulate the local magnetic field by using magnets, birds in the lab change their behaviour by changing their direction of movement. My research specifically focuses on translating these findings to ways in which birds interact with the Earth's magnetic field out in nature. I want to know how birds may use geomagnetic cues when they are migrating between continents. The possibility that birds can use information from an invisible field to travel thousands of kilometres is nothing short of a superpower!
Do birds have nostrils that they can use to smell? Yes! They are called nares and are located at the point where the beak is connected to the rest of the bird's face. Scavenging birds like Turkey Vultures use their sense of smell to locate prime meals like roadkill near highways. According to research, birds might have a "smell map" and know the mix of smells specific to important locations, such as their home range. In terms of migration, researchers have done experiments and found out that birds who have a limited sense of smell can't navigate as effectively as their counterparts who have a fully functioning sense of smell. This suggests that having an intact olfactory system may be necessary to perform the marvellous act of migration.
It might be easy for a trained human eye to tell the difference between a forest, a field, a farm, and a freeway. On a smaller scale, birds use simple landscape features to keep them on track during migration. One possibility is that birds have a general, patchy map of key areas like their home range. It is also possible that over time and with enough interaction with the same landscape features, birds have a more "Google Maps" sense of direction. This means that they know exactly how one landscape feature is connected to the next one. So that when they fly over the roof of House #1, they see the roof of House #2, head in that direction, and then see the roof of House #3 to know where to keep going. In this way, they could be "piloting" their way home.
When a gaggle of Canada geese are gearing up to charge at a friendly passerby, it is not hard to imagine that this group somehow has a plan of attack ready to go! Trying to understand how animals make decisions in groups, however, is no easy feat because there are so many unknown and uncontrollable variables. This is where tools like coding and mathematical models help. Researchers can build software programs to answer questions like: "Is it the adult birds that make most of the group decisions?"; "Do groups consider the greenness of a landscape to decide where they need to take a break?" or "Do young birds switch groups more often that adult birds?" It would be next to impossible to test these questions in a lab. With coding, however, we now know that birds like barnacle geese (a close relative of the brant geese we have here in Canada) might be adapting to climate change during migration by making and following group decisions, often directed by the older, more experienced birds.
There's a reason why early seafarers used the stars to navigate: a clear night sky moves and changes in predictable ways that can be used to roughly pinpoint location and direction of travel. It's easy to see why this navigation cue may also be useful for birds looking for reliable patterns to make migratory decisions. Very early research from the 1940s tested this theory in a planetarium with Indigo buntings. They found that when the orientation of the planetarium sky was flipped (shifted 180o from the natural sky outside), the bird's direction of travel flipped as well. This suggests that birds might indeed be using the star patterns to navigate. In fact, birds may also use other celestial cues like the position of the sun to make sure they are on the right track. Work with Monarch butterflies, another astounding migratory species, certainly points in this direction!
These are just the migration strategies that we know about. It is possible that there are numerous other mysterious cues birds could be using to navigate. Choice of cue may depend on the bird species and migration destination. Research from labs like mine and those of my fellow colleagues are finding new ways to test multiple cues at the same time. This approach provides a more realistic model of how birds successfully complete the marathon of migration year after year. In the meantime, we can sit back in awe as our feathered friends find their ways back to our yards this spring!
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